Ethernet Frame

The minimum size for an Ethernet frame (Header + Payload + Trailer) is 64 bytes.

The minimum sizer for a payload is 46 bytes.

If the payload is less than 46 bytes, padding bytes are added (all zeros).

Header

Preamble

Length: 7 bytes (56 bits)

Allows devices to synchronize their receiver clocks

Bit pattern: 10101010

SFD

Start Frame Delimiter

Length: 1 byte

Markes the end of the preamble and the beginning of the rest of the frame.

Bit pattern: 10101011

Destination and Source Address

The Layer 2 address of the destination device.

MAC address

• Media Access Control

• 6 byte (48 bits)

Type or Length

Length: 2 bytes

A value of 1500 or less in this field indicates the LENGTH of the encapsulated packet (in bytes).

A value of 1536 or greater in this field indicates the TYPE of the encapsulated packet (usually IPv4 or IPv6), and the length is determined via other methods.

Type Value

• IPv4: 0x0800

• IPv6: 0x86DD

• ARP: 0x0806

Trailer

FCS

Frame Check Sequence.

Length: 4 bytes.

Detects corrupted data by running a 'CRC' algorithm over the received data.

CRC = Cyclic Redundancy Check.